Membranes Flashcards
Cell membranes are composed of phospholipids.
What are these phospholipids composed of?
- Glycerol backbone
- 2 OH groups esterified to backbone
- Fatty acid groups
For glycerol based phospholipids, what is special about the third glycerolic hydroxyl group?
Esterified to a phosphate (which is esterified to head group)
Describe the polarity and solubility of fatty acid tails.
- Nonpolar due to lack of charged groups
- Dissolve poorly in polar solvents but well in organic solvents
- Hydrophobic
Describe the solubility of the head groups of phospholipids.
- Charged and water soluble
- Hydrophilic
How do the phospholipids arrange themselves when mixed in water?
- Hydrophobic fatty acids don’t contact with water
- Hydrophilic heads dissolve in water
- At low concentrations, form monolayers. At high concentrations, form micelles.
Describe the phospholipid bilayer.
- Hydrophilic head groups orient towards water
- Hydrophobic tails orient away from water
- Tail length determines bilayer width. Head groups determine how tightly packed molecules are
What does it mean for the structure of the bilayer to be ‘fluid’?
Phospholipids move within the layer in which it resides
How does temperature influence the movement of phospholipids within the bilayer?
- At high temperature - sol state. Rapid lateral diffusion
- At low temperature - gel state. Slow diffusion of phospholipids
- Temperature at which bilayer converts from gel state to sol state is the transition temperature
How do the characteristics of the phospholipid influence movement within the bilayer?
- Saturated fatty acids are closely packed together so require greater thermal energy to permit diffusion.
- Shorter chains and double bonds aren’t as packed so less energy required.
How does changing the type of phospholipid influence fluidity of the membrane?
- Fatty acid tails are different lengths and organisations
- Interact with one another to influence moevment and rigidity
What are the 2 major groups of phospholipids?
Glycerol based
Sphingolipids
What is the effect of having cholesterol at low concentrations?
- Steroid rings bind to and partial immobilisation of fatty acid tails.
- Membrane rigidity increases
- Greater gel sol transition temperature
What is the effect of having cholesterol at high concentrations?
- Disrupts interactions between phospholipids
- Greater fluidity
- Decreased gel-sol transition temperature
Describe the zonation within the bilayers.
- Composed of several lipid types spread over a wide temperature range
- Long chain saturated fatty acids - usually in gel state
- Short chain unsaturated fatty acids - sol state
What are the physiological functions of membranes?
- Separates intracellular fluid/cytosol from extracellular fluid
- Regulation of exchange with environment e.g uptake and removal
- Detects chemical messengers arriving at cell surface
- Anchors cells to cellular matrix
